1. Field of the Invention
This invention relates to a combustion process for reducing the amount of nitrogen oxides generated in combustors, and more particularly it relates to an improvement in multi-stage combustion processes.
2. Description of the Prior Art
Various nitrogen oxides such as NO, NO.sub.2, N.sub.2 O.sub.3, etc., referred to hereinafter as NO.sub.x, have been exhausted from combustors employing fossil fuels, and they have been becoming a portion of atmospheric pollution substances.
NO.sub.x reducing processes employed so far are roughly classified into the following five processes:
processes for reducing so-called thermal NO.sub.x by lowering combustion temperature through (1) mixing of exhaust gas, (2) multi-stage combustion or (3) flame division; (4) processes for reducing so-called fuel NO.sub.x through fuel conversion; and (5) processes for reducing NO.sub.x into harmless N.sub.2 by means of catalysts, hydrocarbons, ammonia, etc.
Among these processes, those of (1), (2) and (3) by lowering combustion temperature have a drawback that exhaust gas contains a large amount of dusts consisting mainly of unburnt carbon. Processes of (4) for removing fuel NO.sub.x raise problems such as increase in fuel cost. Further, reduction processes of (5) require a reducing agent-injecting apparatus, formation of catalyst layer, etc., resulting inevitably in making an apparatus of a larger scale and more complicated.